Enable stack soft delete for event persistence

Instead of removing database records when deleting data from stack,
table, mark them as deleted through the deleted_at field. And allow
access to soft deleted records by stack Id.

blueprint event-persistence

Change-Id: Iaddd9ba683deab65a9b2be3cc2e727c34e816dd4
Provide a way to clean up soft deleted data

Add a purge_deleted command to heat-manage. The command takes an
'age' argument, and removes all database records that have been soft
deleted for more than 'age' days. Default to 90.

blueprint event-persistence

Change-Id: I8a62108a907b4e709c91a93a1f7db6c702cec6b5
Merge "Enable stack soft delete for event persistence"
Merge "Provide a way to clean up soft deleted data"